I have always had little idiosyncrasies, doing certain things in order or placing objects just so. I accepted these, never put a name to them or let them assume any importance in my life.

After the births of my first two children, I found that I indulged in these rituals far more than ever before, particularly if either of the children was ill. I studiously avoided germs where possible and bleached, boiled or sprayed any they might meet into oblivion. The lack of control that I felt was slightly counteracted by the steps I took; it made my life complicated sometimes and earned me the label of "Anxious + + +" in my doctor's notes, but it didn't make me unhappy or take over my life.

It was after the birth of my third child that obsessional and intrusive thoughts engulfed me. A recent survey in the Nursing Times suggests that up to 11 per cent of mothers may suffer from post-natal obsessive-compulsive disorder (OCD). I was unaware that this was an illness in its own right. I know my reluctance to discuss the full extent of my disturbed thought processes led to a delay in diagnosis. The lack of awareness about the illness, coupled with shame and embarrassment, means that women are suffering in silence.

Our third child was much deliberated-over and very much wanted. However, for the first year of her life, I was enslaved by the thought that she might come to harm. I was so overwhelmed by the responsibility of caring for her that my perception of risk became completely skewed.

Fifteen weeks after her birth, I was beginning the inexorable slide into post-natal depression; I was exhausted, yet filled with a malign energy that seemed to leach out of me. I was already experiencing obsessive thoughts about her becoming ill. I rarely left her side and was constantly checking her to ensure that she was OK.

Around the same time, I was carrying her downstairs when I stumbled on the penultimate step, hastily grabbing the wall to right myself. I stood at the foot of the stairs and realised that if I had fallen, nobody would have known until I failed to arrive at school to collect my other children. I could have crushed her, or knocked myself unconscious, leaving her at the mercy of our dogs, an intruder or some other foe, defenceless as I lay out cold on the hall carpet. This thought was accompanied by a flood of adrenalin, which appeared to validate this fear as being important. And that was the start of it. During the next few days, that fear, that thought, inveigled its way into my desperately tired brain, over and over again. I would see myself tumbling down the stairs, crashing into the wall, baby clutched in my arms as I careered downwards. It was like watching a sequence of film repeated time and time again, never losing its clarity, the accompanying bolt of panic never losing its impact either.

Over those days, I began to wonder how I could explain to my husband that I had decided to live downstairs. I believed if I could avoid the stairs, I could avoid the thought and the subsequent fear.

But soon my brain grew bored with this scenario and began to invent other horrors for me to contend with. I began to worry about falling with her or accidentally dropping her at any time. Bridges and multi-storey car parks filled me with terror as I perceived these as particularly dangerous places. However, my brain was clever, dishing up the most abhorrent eventuality as something that could happen right at home. Endlessly, I was tormented by the image of my tiny baby daughter suspended about 30 centimetres from the hallway floor, mid-fall, hung in suspended animation, and I was powerless to stop the inevitable impact.

It was horrendous, terribly wearing. I subsisted on scant food and adrenalin, feeling too nauseous to eat, too anxious to sleep, believing that I would never escape this awfulness. I never felt angry with or resented my daughter. I just felt terribly sorry for her, sorry that she had been born to such a sub-standard mother. I sang to her, read to her and cared for her meticulously, attempting to make up for the deficit of the soul that I felt inside. I would sing nursery rhymes to her, shoulders hunched with tension while the relentless picture show played on inside my head. Night-times were worst, my beautiful daughter snuggled next to me as I lay awake, the thoughts endlessly washing over me; there was no respite, even as all around me slept peacefully.

I decided to ask for counselling. It was at the preliminary assessment that I finally discovered what, apart from the depression, was wrong with me. My daughter bounced on my knee and emptied my handbag, unaware of the torrent of horror pouring from my lips. My assessor told me I had post-natal OCD, explaining that, when suffering from this, a person's brain finds the most awful catastrophe that it can and then proceeds to torture that person with it.

He assured me that I was not mad or bad, just suffering from an illness. My relief was huge. Upon returning home, I looked on OCD-UK's excellent website and found the exact description of my symptoms. Its self-help advice began my recovery. I had to take ownership of OCD, and tell myself that these were "just thoughts… weightless and immaterial". Resisting the thoughts is the very thing that ensures their persistence; by naming and acknowledging them, the familiar stab of adrenalin began to diminish. Coupled with the cognitive behavioural therapy I later received, I was able finally to take control of the situation. It was a confidence trick at first, but soon the thought would come, I would recognise it for what it was and then go and do something else. Never have I walked so much or drunk so much tea.

So what causes maternal OCD? Ashley Fulwood of OCD-UK says: "We are still not entirely sure why post-natal OCD occurs, research is still ongoing, but for many mums with pre-existing, but perhaps less severe, OCD, the OCD becomes more problematic simply because of the normal hormonal changes, anxiety and worrying for a newborn baby that every mother feels."

Research conducted by the US academics Fairbrother and Abramowitz in 2007 suggests that "the perinatal period lowers the threshold for OCD development/exacerbation by bringing with it a sudden increase in responsibility for a vulnerable and highly cherished infant". This can then lead to "the misinterpretation of normally occurring upsetting or intrusive infant-related thoughts as highly significant". Once this has occurred, "behavioural patterns develop in response to intrusive thoughts" which "contribute to the maintenance of obsessional stress".

My recovery was a slow and gradual process. My recovery was like sloughing off a skin, the thoughts began to abate and eventually I could step away and shed them for ever. Since my illness, I have gained much understanding, of both depression and OCD, that has proven transformative. I appreciate the things that I hold dear much more than before. The bond that I have with my daughter is as strong as it possibly could be. I also have far more tolerance and understanding of the havoc these illnesses can wreak. I believe that many people regard OCD as something that provokes hand-washing, excessive tidiness or avoidance of certain things. I did not know that intrusive and unwanted thoughts were part of the illness at all. I still sometimes find myself performing a little ritual at times of stress, but I now consider myself to be fully recovered from an illness that I didn't even realise existed.